This invention relates broadly to a method for making electrical connections involving thin film material and to the electrical connection formed thereby. This invention is especially useful in the flowmeter of the type described and claimed in the U.S. Pat. No. 4,164,865 entitled, "Acoustical Wave Flowmeter" by L. G. Hall and R. S. Loveland, which issued Aug. 21, 1979 and the flowmeter of the U.S. Pat. No. 4,003,252 entitled, "Acoustical Wave Flowmeter" by E. J. DeWath, which issued Jan. 18, 1977.
A simplified reproduction of one of the figures of the Hall and Loveland U.S. Patent drawings is reproduced herein for convenient reference and identified as FIG. 1, Prior Art. The Hall and Loveland system eliminated all impediments to the flow path of the fluid, all cavities in which debris might collect, measured flow accurately regardless of changes in fluid composition or temperature, and determined a change in velocity of sound of the fluid being measured as a function of fluid density.
In order to accomplish this, the Hall and Loveland acoustical wave flowmeter 10 had two, spaced apart, crystal transducers 12 and 14 surrounding the wall or flowmeter conduit 16 (sometimes called a cavity) in in the support ring 18 to produce ultrasonic acoustic compressions at selected frequencies in the fluid within the cavity. The inside and outside surfaces 20-26 of the transducers, shown connected by electrical conductors 28-34 to electronic circuitry 36, were alternately switched by the circuitry 36 into a transmit and a receive mode to generate upstream and downstream transmitted and received signals with an automatic means to adjust the transmitted frequencies to compensate for changes in velocity of the acoustic compressions in the fluid caused by changes in fluid composition and temperature. The electronic circuitry 36 also included, among other things, means for measuring and storing signals representing the phase difference between the transmitting transducer signal which produced compressions and the signal produced by the receiving transducer in response to such compression during each of two successive transmit/receive cycles and processed these signals to indicate flow and change of velocity of the sound traveling through the fluid being measured.
The transducers 12 and 14, represented in FIG. 1, and shown rather thick, are of the crystal type. This type of transducer was found to have a high Q and to be self resonating at the frequencies of interest. This limited the range of frequencies usable in the transducer. Film type transducers also referred to in the U.S. Patents, supra, were found, not only to be lighter in weight, but to have a wider band width and thus increased the usable frequency for the transducer. These films, however, are formed of relatively thin coated piezo film approximately 1,000 A.ANG. thick. Being so thin, the problem of good electrical connection between the electrical conductors and these surfaces becomes difficult to achieve.
The criteria is that electrical connections to the coated film must not damage the film coating physically, and must contact the coating over a large area for low current density for the maximum transfer of electrical energy between the coating and the conductors, and must contact both ends of the coating to minimize conduction delay of energy at ultrasonic frequencies along the length of the thin foil due to significant distributed resistances and capacitances--the so-called delay line effect. Existing techniques utilize conductive epoxy connections and direct metal-to-metal connections between the film and the conductors. These techniques sometimes cause fracturing of the coating due to local high stress areas and cause high current densities due to non-uniform contact areas.